Adds Proton C Conversion (#4661)

* adds proton c base

* fixes custom matrix include

* adds readme.md

* initial proton coversion, no pin mapping

* start of mcu selection

* add pin mapping and sweet16 test

* add at90 to list

* disable backlight, fix d7

* update flag names

* doc updates

* proton c update for mcu selection

quantum/stm32/proton_c.mk

@@ -0,0 +1,44 @@
+# Proton C MCU settings for converting AVR projects
+
+# These are defaults based on what has been implemented for ARM boards
+AUDIO_ENABLE = yes
+RGBLIGHT_ENABLE = no
+BACKLIGHT_ENABLE = no
+
+# The rest of these settings shouldn't change
+
+## chip/board settings
+# - the next two should match the directories in
+#   <chibios>/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)
+MCU_FAMILY = STM32
+MCU_SERIES = STM32F3xx
+
+# Linker script to use
+# - it should exist either in <chibios>/os/common/ports/ARMCMx/compilers/GCC/ld/
+#   or <this_dir>/ld/
+MCU_LDSCRIPT = STM32F303xC
+
+# Startup code to use
+#  - it should exist in <chibios>/os/common/startup/ARMCMx/compilers/GCC/mk/
+MCU_STARTUP = stm32f3xx
+
+# Board: it should exist either in <chibios>/os/hal/boards/
+#  or <this_dir>/boards
+BOARD = GENERIC_STM32_F303XC
+
+# Cortex version
+MCU  = cortex-m4
+
+# ARM version, CORTEX-M0/M1 are 6, CORTEX-M3/M4/M7 are 7
+ARMV = 7
+
+USE_FPU = yes
+
+# Vector table for application
+# 0x00000000-0x00001000 area is occupied by bootlaoder.*/
+# The CORTEX_VTOR... is needed only for MCHCK/Infinity KB
+# OPT_DEFS = -DCORTEX_VTOR_INIT=0x08005000
+OPT_DEFS =
+
+# Options to pass to dfu-util when flashing
+DFU_ARGS = -d 0483:df11 -a 0 -s 0x08000000:leave